Heretofore, a methacrylic acid ester is industrially produced by an esterification reaction between a methacrylic acid and an alcohol or a phenol in the presence of an acid catalyst, or by an ester-exchange reaction between a methacrylic acid ester and an alcohol or a phenol in the presence of an ester-exchange catalyst. In recent years, the method of producing the methacrylic acid ester by the ester-exchange reaction is often used, because both of a starting material and a product are an ester and an alcohol or a phenol, and hence, the products are relatively easily separated by distillation and waste water is not generated.
As the ester-exchange reaction, a reaction using methyl methacrylate as shown in the following formula (1) is frequently employed:
wherein R represents an alkyl group, an aryl group, an alkenyl group, or an aralkyl group.
However, the ester-exchange reaction shown by the above formula (1) is an equilibrium reaction, and hence, it does not proceed beyond a certain conversion. When the conversion is low, it is necessary to separate all the ingredients of a raw methyl methacrylate (A), a raw alcohol (B), an end product methacrylic acid ester (C), and by-product methanol (D) after the reaction, which makes the operations complicated.
Therefore, in order to shift the equilibrium to the product side and further enhance the conversion, it is conducted to remove by-product methanol (D) together with an azeotropic solvent from the reaction system. For example, since raw methyl methacrylate (A) forms an azeotropic mixture with by-product methanol (D), a charge mole number of raw methyl methacrylate (A) is set so as to be in excess of a charge mole number of the raw alcohol (B), and by-product methanol (D) is removed as an azeotropic mixture with methyl methacrylate (A) from the reaction system.
However, in a case where the reaction rate, i.e., the production rate of by-product methanol (D) is not sufficiently high, a large excess of methyl methacrylate (A) is necessary to remove by-product methanol (D), so that the productivity tends to be poor.
Accordingly, in order to form an azeotropic mixture in which a concentration of by-product methanol (D) is enhanced and to remove it from the reaction system, there is employed a method wherein a portion of the azeotropic mixture is distilled out and the remainder is refluxed while controlling a reflux ratio, by the use of a distillation column. In this method, it is desired to produce the methacrylic acid ester with a higher productivity.
For example, Japanese Patent Laid-Open Nos. 87747/1980, 115850/88 and 118352/1991 contain processes for producing an intended methacrylic acid ester by an ester-exchange reaction between methyl methacrylate and an alcohol or a phenol by the use of a reactor equipped with a distillation column, wherein the ester-exchange reaction is carried out while extracting an azeotropic mixture of methyl methacrylate and methanol under the control of only a column top temperature of the distillation column.